If I was in, say, Nevada, which is part of the 9th circuit court, I'd lay down that case in the opening argument of the trial court and say: I win. Anyway, I'd get free legal defense since this would be a criminal case.
They could appeal it. Ok. The appeals court would probably rule the same way as that is what they're supposed to do.
They could bring it all the way to the supreme court. I could put on my resume that I argued a case in front of SCOTUS.
You're right in that nothing will really prevent a law enforcement agency from arresting me, you, or anyone. But attempting to prosecute when there's a clear ruling against them? Unlikely.
MANY NFA dealers and non-registered firearms owners thought that way and went to federal prison over their "principles". The ATF has a special dispensation towards imprisoning those who attempt to test their authority. It's easily referenced on the interwebz, just google the subject and you'll find tons of info. They tried VERY hard to get Doug Friesen Esq. in OKC. It cost him a lot of time, money and effort to ultimately plead out to a misdemeanor "failing to keep accurate records as an FFL".
Google "Johhnie lawmaster ATF case". He didn't do anything wrong at all and look at how it turned his life upside down.
It's one thing to challenge an ATF legal position in the courts outright. It's another entirely to do so as a defendant. NEVER get behind the 8 ball if you can help it!